This invention relates to automatic calibration of a controller for control of a plant.
In the past, AC motor controls have applied balanced three phase voltages to the three stator phases based on the electromechanical characteristics of the motor and on an equivalent circuit model for the motor in the steady state. Therefore, the desired performance characteristics were achieved only in the steady state and control of the the AC machine in the transient state was less precise than that of a DC machine.
With the advent of Field Oriented Control (FOC) of three phase AC machines, the ability to very precisely and separately control transient and steady state quantities of torque and flux in AC machines has become possible. As a result, the AC machine gains the advantages of a DC machine without the drawback of mechanical commutation. FOC is based on three major points: the machine current and voltage space vectors, the transformation of a three phase speed and time dependent system in to a two-coordinate (d, q) time invariant system and effective pulse width modulation pattern generation. Due to advancements in modern semiconductors in both power and signal electronics, precise control of these points has become possible. FOC has become well known in the art of 3 phase AC machine control.
In FOC control, it is known to use proportional-plus-integral (PI) closed loop control of motor current to provide synchronous control of three phase AC motor currents. In order for a PI control to accurately control the AC motor currents, it must be precisely tuned to the electromagnetic dynamics of the motor. In particular, for a PI controller, the proportional gain and the integral gain should be matched to the inductive and resistive characteristics of the particular three phase AC motor to be controlled to provide accurate control of the AC motor currents. The more precisely the controller gains are tuned to the particular motor, the more precisely the motor can be controlled. However, because no two electrical motors have exactly the same electromagnetic characteristics, each controller must be individually tuned to the motor it is intended to control.
One method to used is to determine the motor equivalent inductance by applying a pulse to two phases of the motor and estimating the inductance from the equation L=V/(di/dt) where V is the DC bus voltage and (di/dt) is the rate of rise of the current. This method, however, has been shown to produce errors due to parasitic high frequency effects in the motor. The stator resistance is measured by applying a low DC voltage to the stator and measuring the resultant current.
Another method of determining the electromagnetic characteristics of an AC machine is disclosed in U.S. Pat. No.5,880,415 (the ""415 patent). The disclosure of the ""415 patent teaches a method for calculating a proportional gain, an integral gain of an integrator, and an overall gain for an elevator motor controller current regulator compensation, the controller and motor forming a current loop, including: a) minimizing the contribution of the integrator to the controller during steps (b)-(f); b) setting the proportional gain to an initial value; c) setting the overall gain based on a first test frequency; d) providing a sinusoidal current reference signal to the current regulator at the first test frequency; e) calculating an open loop gain of the current loop at the first test frequency; f) varying the proportional gain and performing steps (e) until the open loop gain is within a predetermined tolerance of 1; g) providing the sinusoidal current reference signal to the current regulator at a second test frequency; h) calculating a closed loop gain of the current loop at the second test frequency; and i) varying the integral gain and performing step (h) until the closed loop gain is within a predetermined tolerance of 1. This method however applies only a single frequency sinusoid to the circuit and uses an iterative technique to tune the controller.
The present invention discloses a circuit and method for calculating a first and second gain for a controller circuit for a plant comprising the steps of providing a reference model circuit with desired circuit characteristics, providing a reference model adaptive circuit, setting the second gain to zero, supplying a spectrally rich input signal to the reference model and to the controller circuit, determining the first gain by continuously adjusting the first gain until an output of the reference model and an output of the controller circuit are substantially equal, setting the first gain to the previously determined value, determining the second gain by continuously adjusting the second gain until an output of the reference model and an output of the controller circuit are substantially equal, and setting the second gain to the previously determined value.
The invention represents a significant improvement over the prior art by allowing the controller gain parameters to be automatically determined. Thus, the invention greatly reduces cost associated with tuning the controller to a plant and while providing a more finely tuned result.